Slavery was addressed in the formation of the United States. When South Carolina left the Union they did so due to their belief that the laws of the United States, particularly as they concerned slavery were being ignored and flaunted by the States of the North. So in consequence they chose to leave the Union, which they believed to be their right under the Constitution.
Make no mistake about it, the "Lost Cause" notwithstanding the war was fought for States Rights and the right of the States to legally enforce the slave laws. They are joined at the hip, although States Rights do emcompass a wider meaning.
One has to only read the "Succession of South Carolina" to see the intent of the State.
The additional causes, reasonings, given since that time, is only revisionist history. While it is generally acknowledged that much that has been written is from the "victors" point of view, it is also true that the formation of the vanquished States of the Confederacy's doctrine of "The Lost Cause" is also revisionist history.
Prior to the war these United States were a Confederation of States. After the war we became "The" United States, a single entity with a strong central form of government.